Serial data transmission loops have found increasing usage in recent years particularly in the area of terminal-attachment. The major drawbacks of loop communications networks are that they are unreliable and relatively vulnerable. A single loop represents an extreme series configuration. Thus, if a single element fails or if one or more breaks occur in the serial single loop network, the entire network will be disabled. Similarly, when it is desired to add an element to the single series loop, the entire loop network is disabled until the new element is connected. The unreliability of the loop increases drastically as the number of elements on the loop increases.
Modifications to the single loop configuration, with the aim of increasing reliability, are well known in the prior art. Thus, in P. Zafiropulo, "Performance Evaluation of Reliability Improvement Techniques for Single-Loop Communications Systems", IEEE Transactions on Communications, VOL. COM-22, No. 6, June, 1974, p. 742, the author points out three loop-reliability (availability) improvement techniques, all of which are based on using an auxiliary transmission path parallel to the main loop. In the first so-called bypass technique, both main and standby loops transmit in the same direction. When a catastrophic failure occurs, the signal stream is routed onto the standby loop. The second method is the self-heal technique. Here, main and standby loops transmit in opposite directions. When a catastrophic failure occurs, the signal stream is rerouted such that the resulting configuration consists of a loop which bends back on itself at either side of the failure. The combined technique is the third method and consists of ORing the bypass and self-healing techniques, i.e., either the bypass or self-healing action can occur but not simultaneously.
Other methods of increasing the reliability of closed loop communications systems have been proposed. Thus, in E. Hafner et al., "Enhancing the Availability of a Loop System by Meshing", a paper presented at the 1976 International Zurich Seminar, a configuration called a braid was proposed. In such a configuration, an outer circular path serves as the main path containing the terminal access points and inner paths serve as bypasses that can be used for bridging the main path between two switches configured on the outer circular path.
Another method of increasing loop reliability was disclosed by G. J. Laurer et al. in "Automatic Loop Reconfigurator", IBM Technical Disclosure Bulletin, VOL. 19, No. 10, March 1977, page 3824. In such a configuration, a controller in the loop is provided with a mechanism for detecting a break or failure of a component in the loop. When this condition is detected, the controller divides the network into two subsystems and initiates a number of steps which result in the creation of two half duplex subsystems, each of which originates at the controller and terminates at one of the two ends created by the break or failure in the loop.
One implementation of the self-healing technique discussed by Zafiropulo, op. cit., is that disclosed in U.S. Pat. No. 3,652,798, issued to Joseph Hood McNeilly et al. on Mar. 28, 1972. In the McNeilly system, a timing station provides time division multiplex channel signals on a first closed loop unidirectional transmission line interconnective in tandem subscriber stations, each of which may gain access to an unused channel signal for communication with an idle subscriber station. To protect against failure of the entire system due to a break in the line or failure in one of the subscriber stations, a second closed loop unidirectional transmission line is connected to all stations transmitting signals in a direction opposite to that on the first line. Each subscriber station can detect an error and transfer the communications signals on the first line to the second line. The subscriber station before the break transfers the communication signal to the second line and the subscriber station after the break transfers the communication signals back to the first line to form a new, but continuous closed loop. When communication signals are on the second line and a fault occurs, the transfer of communications signals will be similarly performed to provide still another new, but continuous closed loop bypassing the fault.
In the McNeilly system, when a first subscriber station next to a fault detects the absence of signals on the first (primary) line input,, it simultaneously wraps its secondary output to its primary input and breaks its output connection to the secondary line to isolate the subscriber station from the primary and secondary lines on the fault side of the subscriber station. At the same time it sends an alarm signal out over the primary line to the next subscriber station. The next subscriber station detects initially the absence of signal on the primary line and operates to break the two lines and connect its secondary output to its primary input as in the case of the first subscriber station. However, the next subscriber station is still able to receive the alarm signals from the first subscriber station and when this is received the secondary output to primary input connection is broken and the next subscriber station reverts to normal operation. This procedure is repeated until the alarm signal reaches the (last) subscriber station on the other side of the fault (which had initiated error recovery when the first subscriber station broke its secondary output, thus causing a loss of signal at the secondary input to the last subscriber station), where it is transferred to the secondary line and so eventually reaches the timing station for the second time, having once passed through the timing station on the primary line. The timing station then removes from its outgoing secondary line the unique standby signal and connects the outgoing line directly to a bypass connection in the timing station. The timing station therefore includes a standby signal generator, a primary line signal detector and a bypass switch. Thus, after a brief pause, all subscriber stations are again connected by a new unbroken closed loop composed of the primary and secondary lines on both sides of the fault and loop backs implemented by each subscriber station adjacent to the fault.
The McNeilly system has several inherent drawbacks. First, the system utilizes an alarm signal to reset subscriber stations which incorrectly initiate error recovery when a fault occurs. Thus, each subscriber station must include detection circuitry to differentiate between the receipt of normal signals and an alarm signal. Second, the system requires timing station circuitry to detect the loss of the standby signal and in response disconnect the secondary line signal generator. The timing station must therefore be able to distinguish between the primary and secondary line signals on the secondary input to the timing station. A third drawback of the system is that upon detection of a loss of primary input signal at a subscriber station, the subscriber station breaks its secondary line output simultaneously with wrapping its secondary output to its primary input. Thus, a secondary line fault is created before it is determined whether the primary signal loss is due to an actual break in the primary line adjacent to the subscriber station or a break in the primary line adjacent to some other subscriber station. Thus, incorrect error recovery due to the created secondary line fault is initiated. This will cause an increase in the time required for the system to recover from the actual break in the primary line as subscriber station will have to be reset a second time.
It is a general object of the present invention to eliminate these and other drawbacks of the prior art by providing an improved self-healing loop communications network.
It is another object of the present invention to provide a self-healing loop communications network which does not require a central control for accomplishing error recovery.
It is still another object of the present invention to provide a self-healing loop communications network wherein all recovery actions are accomplished under strictly local control.
It is a further object of the present invention to provide a self-healing loop communications network wherein subscriber stations directly involved in detecting a loop failure require no direction from other subscriber stations to complete the error recovery process.
These and other objects, features and advantages of the present invention will become more apparent from the detailed description of the preferred embodiment when read in conjunction with the drawings.